(1) Field of the Invention
A method of preparing 2,4-difluoroaniline.
(2) Description of the Prior Art
G. C. Finger and C. W. Kruse, in JACS, 78, 6034 (1956), have described fluorination of 2,3,4-trichloro nitrobenzene with potassium fluoride in dimethylsulfoxide, to form 1,3-difluoro-2-chloro-6-nitrobenzene, but in low yield. Finger et al., in JACS, 81, 94 (1959), have described fluorination of 3,4,5-trichloronitro-benzene with potassium fluoride in dimethylformamide, to form 3,5-dichloro-4-fluoronitrobenzene in fairly high yield. However, the art does not suggest the specific fluorination by chlorine replacement to form 2,4-difluoro-5-chloronitrobenzene achieved with the method of the present invention, nor the high yield efficiency achieved with phase transfer catalysis in said method.
U.S. Pat. No. 3,992,432 describes a process for catalyzing heterogeneous ionic organic reactions in a system of multiple liquid phases in which the reactants are located in different phases. The catalysts are certain organic quaternary salts. However, the method of the patent is one involving only liquid phases, especially an organic phase and an aqueous phase, and there is no suggestion of employing a solid phase and a liquid phase as in the method of the present invention.
Christensen et al., in Chem. Res., 74, 351 (1974), and Gokel and Durst, in Aldrichimica Acta, 9, 3 (1976), describe the use of crown ethers to catalyze a variety of synthetic reactions by complexing with an insoluble reagent, rendering the entire complex soluble in organic solvents. However, the crown ethers suffer from the disadvantages of limited solubility, cation specificity, and high cost; none of which characterize the phase transfer catalysts of the present invention.
Hydrogenolysis of various halobenzenes with hydrogen over palladium-carbon by M. Kraus and V. Bazant, reported in Catal., Proc. Int. Congr., 5th, 1972, 2, 1073-84, determined that reactivity decrease occurred in the order Br, Cl, F. and R. E. Florin et al., in J. Res. Natl. Bur. St., 62, 119 (1959) found that hydrogenation with hydrogen over palladium-carbon of 2,3,4,5-tetrafluorochlorobenzene formed 1,2,3,4-tetrafluorobenzene in fairly high yield. However, when catalytic reduction over palladium black of aromatic fluoro chloro nitro compounds was carried out by N. Vorozhtsov et al., reported in Zhur. Obshchei Khim., 31, 1229-32 (1961), it was found that 2,4-difluoro-5-chloroaniline was formed. Thus, the art does not suggest the catalytic reduction of the method of the present invention whereby the nitro group is reduced to amine and the chlorine substituent is displaced without affecting the fluorine substituents, in a very specific manner.
A commonly employed method in the art currently for preparing 2,4-difluoroaniline is one which prepares 1,3-dichlorobenzene as an intermediate and uses benzene as a starting material. However, this process requires use of AlCl.sub.3 catalyst, an expensive material, as well as fractional distillation. The intermediate is then nitrated, and fluorine replacement and reduction produces 2,4-difluoroaniline. The reactions of this conventional method may be illustrated as follows: ##STR1## The method of the present invention avoids the expensive isomerization step of the above process by employing trichlorobenzene, as inexpensive starting material, with specific chlorine removal at the time of fluorine replacement.